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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 27 — Dec. 17, 2012
  • pp: 28941–28946
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Experimental demonstration of passive coherent combining of fiber lasers by phase contrast filtering

François Jeux, Agnès Desfarges-Berthelemot, Vincent Kermène, and Alain Barthelemy  »View Author Affiliations


Optics Express, Vol. 20, Issue 27, pp. 28941-28946 (2012)
http://dx.doi.org/10.1364/OE.20.028941


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Abstract

We report experiments on a new laser architecture involving phase contrast filtering to coherently combine an array of fiber lasers. We demonstrate that the new technique yields a more stable phase-locking than standard methods using only amplitude filtering. A spectral analysis of the output beams shows that the new scheme generates more resonant frequencies common to the coupled lasers. This property can enhance the combining efficiency when the number of lasers to be coupled is large.

© 2012 OSA

1. Introduction

Coherent combining techniques have been widely studied these last years to increase the brightness of fiber lasers. This kind of laser source is based on the use of several parallel amplifiers. The multiple output beams from these amplifiers have to be locked in phase so that their coherent summation occurs in the far field where all the beams overlap. The coherent control of the laser outputs provides high peak power, evolving with the square of the number of amplifying channels, on the propagation axis. The coherent combining techniques are based either on active or on passive phase-locking methods depending on whether the phase control is made in a MOPA architecture or inside a compound cavity. The main interest of passive techniques is their capability to self-adjust the laser array operation to maintain in phase relationships in an unprotected environment. This kind of laser sources consists in a single cavity including several parallel arms with amplifiers. Intra cavity coupling and filtering select the inphase emission while the laser spectrum self-adjusts to minimize losses. Different laser architectures, which are simple to implement, operate on this principle and have achieved coherent power combining [1

1. L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Petersson, “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area,” Opt. Lett. 30(13), 1668–1670 (2005). [CrossRef] [PubMed]

10

10. M. Fridman, M. Nixon, N. Davidson, and A. A. Friesem, “Passive phase locking of 25 fiber lasers,” Opt. Lett. 35(9), 1434–1436 (2010). [CrossRef] [PubMed]

]. Unfortunately, it is difficult to efficiently lock a large array of lasers. Considering fiber lasers, the decrease of combining efficiency becomes sensitive beyond 10 lasers [10

10. M. Fridman, M. Nixon, N. Davidson, and A. A. Friesem, “Passive phase locking of 25 fiber lasers,” Opt. Lett. 35(9), 1434–1436 (2010). [CrossRef] [PubMed]

13

13. W.-Z. Chang, T.-W. Wu, H. G. Winful, and A. Galvanauskas, “Array size scalability of passively coherently phased fiber laser arrays,” Opt. Express 18(9), 9634–9642 (2010). [CrossRef] [PubMed]

]. It can be explained by the fact that the elementary fiber lasers cannot be built and maintained with identical lengths. So common resonance frequencies become scarce in the gain bandwidth when the number of sub-cavities increases, up to the case where there is not even a single frequency compatible with an inphase emission. Numerical and theoretical approaches have been shown to reproduce the observed decrease in efficiency. The limitation applies to almost all the known passive techniques.

A large number of passive techniques for fiber lasers are based on couplings leading to a uniform feedback in the different amplifying channels. As the feedback seeds all the parallel amplifiers with the same amplitude and phase information, the wavelength is the only degree of freedom of the laser to ensure the delivery of phase-locked fields despite the differences in sub-cavity lengths. We have recently proposed a new architecture exploiting nonlinear phase contributions and individual feedbacks to the parallel amplifying sections [14

14. F. Jeux, A. Desfarges-Berthelemot, V. Kermène, J. Guillot, and A. Barthelemy, “Passive coherent combining of lasers with phase-contrast filtering for enhanced efficiency,” Appl. Phys. B 108(1), 81–87 (2012). [CrossRef]

]. It increases the degrees of freedom of the compound laser. The working principle can be summarized as follow. Let us assume first that the parameters of the multi-branch laser are such that there is no resonance frequency shared by the different sub-cavities of the resonator. For the frequency of lowest loss, the laser delivers an array of beams which are not perfectly synchronized. Considering approximately identical intensity for the exiting elementary fields, their phase deviations are mapped into amplitude deviations before they are returned for a new round in the cavity. The resulting inhomogeneous distribution of feedbacks creates differences in gain among the amplifiers. Saturation of the amplification is therefore not identical in all the laser arms. The resonant contribution to the refractive index due to the gain adds a phase-shift which mitigates to some extent the linear phase deviation. After iteration of the process at each cavity round trip in the compound cavity, the multiple output beams converge to a steady state inphase emission at the expense of slight intensity modulation in the array cross-section. We numerically demonstrated, in reference [14

14. F. Jeux, A. Desfarges-Berthelemot, V. Kermène, J. Guillot, and A. Barthelemy, “Passive coherent combining of lasers with phase-contrast filtering for enhanced efficiency,” Appl. Phys. B 108(1), 81–87 (2012). [CrossRef]

], the capability of this new scheme of cavity to improve combining efficiency for large number of parallel amplifying channels. It was suggested to perform the phase to intensity mapping by means of a phase contrast imaging set-up.

We report here preliminary experiments on the implementation of the phase-contrast technique for the synchronization of four fiber lasers together with comparison with the architecture based on uniform feedback. It was predicted that the new phase-locking process increases the number of oscillating frequencies in the gain bandwidth, fitted to the emission of the in-phase mode. In this paper, we present an experimental setup that highlights this phenomenon. Spectral filling improvement of multi-amplifier lasers is an essential point to overcome limitation on the number of lasers that can be synchronized by passive techniques.

2. Phase contrast laser cavity

Consider first a spatial filter like a pinhole introduced in the central part of the far-field. Phase-locking is obtained in that case by standard filtering rules of passive combining techniques. The laser architecture is then very similar to the all feedback loop laser or to the interferometer laser for which the pinhole is replaced by the input end of a single mode fiber [2

2. D. Sabourdy, V. Kermene, A. Desfarges-Berthelemot, and A. Barthélémy, “Power scaling of fibre lasers with all-fibre interferometric cavity,” Electron. Lett. 38(14), 692–693 (2002). [CrossRef]

,3

3. A. Shirakawa, T. Saitou, T. Sekiguchi, and K. Ueda, “Coherent addition of fiber lasers by use of a fiber coupler,” Opt. Express 10(21), 1167–1172 (2002). [CrossRef] [PubMed]

,6

6. J. Lhermite, A. Desfarges-Berthelemot, V. Kermène, and A. Barthélémy, “Passive phase locking of an array of four fiber amplifiers by an all-optical feedback loop,” Opt. Lett. 32(13), 1842–1844 (2007). [CrossRef] [PubMed]

,7

7. T. H. Loftus, A. M. Thomas, M. Norsen, J. Minelly, P. Jones, E. Honea, S. A. Shakir, S. Hendow, W. Culver, B. Nelson, and M. Fitelson, “Four-channel, high power, passively phase locked fiber array,” in Advanced Solid-State Photonics, Technical Digest (CD) (Optical Society of America, 2008), paper WA4.

]. In such kind of compound resonators, the same amplitude and phase information is back-seeded to all the amplifying arms after filtering. Only the central part of the inner cavity far field is feedback to the whole amplifying arms. The inphase mode exhibiting the highest on axis intensity is best fitted to this filter giving thus the lowest intracavity losses. The laser therefore generates the inphase mode if the gain bandwidth contains the appropriate set of frequencies common to all its sub-cavities.

Previously known passive coherent combining techniques are already efficient to phase-lock a small number of lasers with an appropriate design. We have specifically chosen the lengths of the laser amplifying arms to depart from a good design. So, the configuration of Fig. 1 with a pinhole should have a spectral behavior similar to the one of a well designed laser array of larger size. In that case, common resonant frequencies are becoming scarcer in the compound laser. It is possible to observe such behavior with a four amplifiers laser by adjusting the four sub-cavity lengths close to each other. The number of common resonant frequencies indeed depends on the product of the root mean square length difference δL with the useful spectral bandwidth Δλ of the laser [12

12. J. E. Rothenberg, “Passive coherent phasing of fiber laser arrays,” Proc. SPIE 6873, 687315, 687315-9 (2008). [CrossRef]

]. The maximum length difference ΔL between the four sub-cavities was fixed around 1mm instead of few meters usually, leading to an average spectral envelop structuring δλ greater than 1nm at λo = 1085nm (δλ ~λo2/ ΔL).

3. Phase contrast laser behavior

After adjustment of the cavity the laser system including the APP component has operated on an inphase mode as expected. That was obtained despite the improper choice of fiber amplifiers lengths.

We can see on Fig. 2
Fig. 2 (a) Far field pattern of the 4 output beams from the laser using the APP component. (b) Far field cross section.
, the far field recorded at the laser output, made of a main central lobe and four side lobes in a square distribution, which is typically due to the identical phase of the four output beams. The observation confirmed experimentally that the new filtering process converge to the generation of the inphase mode as it was numerically predicted [14

14. F. Jeux, A. Desfarges-Berthelemot, V. Kermène, J. Guillot, and A. Barthelemy, “Passive coherent combining of lasers with phase-contrast filtering for enhanced efficiency,” Appl. Phys. B 108(1), 81–87 (2012). [CrossRef]

]. The phase-locking performance of the array was estimated to be better than λ/10 after time averaging. In that first experiment on the phase contrast approach, the fiber amplifiers were of low power and the efficiency of the cavity was not optimized. The delivered power was at maximum of 1.5W.

4. Conclusion

Acknowledgments

The authors thank ASTRIUM and CILAS for their support to the present study.

References and links

1.

L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Petersson, “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area,” Opt. Lett. 30(13), 1668–1670 (2005). [CrossRef] [PubMed]

2.

D. Sabourdy, V. Kermene, A. Desfarges-Berthelemot, and A. Barthélémy, “Power scaling of fibre lasers with all-fibre interferometric cavity,” Electron. Lett. 38(14), 692–693 (2002). [CrossRef]

3.

A. Shirakawa, T. Saitou, T. Sekiguchi, and K. Ueda, “Coherent addition of fiber lasers by use of a fiber coupler,” Opt. Express 10(21), 1167–1172 (2002). [CrossRef] [PubMed]

4.

B. Colombeau, M. Vampouille, V. Kermene, A. Desfarges, and C. Froehly, “Spatial shaping of coherent waves inside a confocal laser,” Pure Appl. Opt. 3(5), 757–773 (1994). [CrossRef]

5.

C. J. Corcoran and F. Durville, “Experimental demonstration of a phase-locked laser array using a self-Fourier cavity,” Appl. Opt. 86(20), 201118 (2005).

6.

J. Lhermite, A. Desfarges-Berthelemot, V. Kermène, and A. Barthélémy, “Passive phase locking of an array of four fiber amplifiers by an all-optical feedback loop,” Opt. Lett. 32(13), 1842–1844 (2007). [CrossRef] [PubMed]

7.

T. H. Loftus, A. M. Thomas, M. Norsen, J. Minelly, P. Jones, E. Honea, S. A. Shakir, S. Hendow, W. Culver, B. Nelson, and M. Fitelson, “Four-channel, high power, passively phase locked fiber array,” in Advanced Solid-State Photonics, Technical Digest (CD) (Optical Society of America, 2008), paper WA4.

8.

S. Auroux, V. Kermène, A. Desfarges-Berthelemot, and A. Barthélémy, “Coherence properties of two fiber lasers coupled by mutual injection,” Opt. Express 17(20), 17694–17699 (2009). [CrossRef] [PubMed]

9.

E. Ronen and A. A. Ishaaya, “Phase locking a fiber laser array via diffractive coupling,” Opt. Express 19(2), 1510–1515 (2011). [CrossRef] [PubMed]

10.

M. Fridman, M. Nixon, N. Davidson, and A. A. Friesem, “Passive phase locking of 25 fiber lasers,” Opt. Lett. 35(9), 1434–1436 (2010). [CrossRef] [PubMed]

11.

D. Kouznetsov, J. Bisson, A. Shirakawa, and K. Ueda, “Limits of coherent addition of lasers: simple estimate,” Opt. Rev. 12(6), 445–447 (2005). [CrossRef]

12.

J. E. Rothenberg, “Passive coherent phasing of fiber laser arrays,” Proc. SPIE 6873, 687315, 687315-9 (2008). [CrossRef]

13.

W.-Z. Chang, T.-W. Wu, H. G. Winful, and A. Galvanauskas, “Array size scalability of passively coherently phased fiber laser arrays,” Opt. Express 18(9), 9634–9642 (2010). [CrossRef] [PubMed]

14.

F. Jeux, A. Desfarges-Berthelemot, V. Kermène, J. Guillot, and A. Barthelemy, “Passive coherent combining of lasers with phase-contrast filtering for enhanced efficiency,” Appl. Phys. B 108(1), 81–87 (2012). [CrossRef]

15.

J. W. Arkwright, P. Elango, G. R. Atkins, T. Whitbread, and M. J. F. Digonnet, “Experimental and theoretical analysis of the resonant nonlinearity in Ytterbium-doped fiber,” J. Lightwave Technol. 16(5), 798–806 (1998). [CrossRef]

16.

A. A. Fotiadi, O. L. Antipov, and P. Mégret, “Dynamics of pump-induced refractive index changes in single-mode Yb-doped optical fibers,” Opt. Express 16(17), 12658–12663 (2008). [CrossRef] [PubMed]

17.

C. J. Corcoran and F. Durville, “Passive phasing in a coherent laser array,” IEEE J. Sel. Top. Quantum Electron. 15(2), 294–300 (2009). [CrossRef]

OCIS Codes
(140.3290) Lasers and laser optics : Laser arrays
(140.3298) Lasers and laser optics : Laser beam combining

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: October 18, 2012
Revised Manuscript: November 28, 2012
Manuscript Accepted: November 29, 2012
Published: December 12, 2012

Citation
François Jeux, Agnès Desfarges-Berthelemot, Vincent Kermène, and Alain Barthelemy, "Experimental demonstration of passive coherent combining of fiber lasers by phase contrast filtering," Opt. Express 20, 28941-28946 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-27-28941


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References

  1. L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Petersson, “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area,” Opt. Lett.30(13), 1668–1670 (2005). [CrossRef] [PubMed]
  2. D. Sabourdy, V. Kermene, A. Desfarges-Berthelemot, and A. Barthélémy, “Power scaling of fibre lasers with all-fibre interferometric cavity,” Electron. Lett.38(14), 692–693 (2002). [CrossRef]
  3. A. Shirakawa, T. Saitou, T. Sekiguchi, and K. Ueda, “Coherent addition of fiber lasers by use of a fiber coupler,” Opt. Express10(21), 1167–1172 (2002). [CrossRef] [PubMed]
  4. B. Colombeau, M. Vampouille, V. Kermene, A. Desfarges, and C. Froehly, “Spatial shaping of coherent waves inside a confocal laser,” Pure Appl. Opt.3(5), 757–773 (1994). [CrossRef]
  5. C. J. Corcoran and F. Durville, “Experimental demonstration of a phase-locked laser array using a self-Fourier cavity,” Appl. Opt.86(20), 201118 (2005).
  6. J. Lhermite, A. Desfarges-Berthelemot, V. Kermène, and A. Barthélémy, “Passive phase locking of an array of four fiber amplifiers by an all-optical feedback loop,” Opt. Lett.32(13), 1842–1844 (2007). [CrossRef] [PubMed]
  7. T. H. Loftus, A. M. Thomas, M. Norsen, J. Minelly, P. Jones, E. Honea, S. A. Shakir, S. Hendow, W. Culver, B. Nelson, and M. Fitelson, “Four-channel, high power, passively phase locked fiber array,” in Advanced Solid-State Photonics, Technical Digest (CD) (Optical Society of America, 2008), paper WA4.
  8. S. Auroux, V. Kermène, A. Desfarges-Berthelemot, and A. Barthélémy, “Coherence properties of two fiber lasers coupled by mutual injection,” Opt. Express17(20), 17694–17699 (2009). [CrossRef] [PubMed]
  9. E. Ronen and A. A. Ishaaya, “Phase locking a fiber laser array via diffractive coupling,” Opt. Express19(2), 1510–1515 (2011). [CrossRef] [PubMed]
  10. M. Fridman, M. Nixon, N. Davidson, and A. A. Friesem, “Passive phase locking of 25 fiber lasers,” Opt. Lett.35(9), 1434–1436 (2010). [CrossRef] [PubMed]
  11. D. Kouznetsov, J. Bisson, A. Shirakawa, and K. Ueda, “Limits of coherent addition of lasers: simple estimate,” Opt. Rev.12(6), 445–447 (2005). [CrossRef]
  12. J. E. Rothenberg, “Passive coherent phasing of fiber laser arrays,” Proc. SPIE6873, 687315, 687315-9 (2008). [CrossRef]
  13. W.-Z. Chang, T.-W. Wu, H. G. Winful, and A. Galvanauskas, “Array size scalability of passively coherently phased fiber laser arrays,” Opt. Express18(9), 9634–9642 (2010). [CrossRef] [PubMed]
  14. F. Jeux, A. Desfarges-Berthelemot, V. Kermène, J. Guillot, and A. Barthelemy, “Passive coherent combining of lasers with phase-contrast filtering for enhanced efficiency,” Appl. Phys. B108(1), 81–87 (2012). [CrossRef]
  15. J. W. Arkwright, P. Elango, G. R. Atkins, T. Whitbread, and M. J. F. Digonnet, “Experimental and theoretical analysis of the resonant nonlinearity in Ytterbium-doped fiber,” J. Lightwave Technol.16(5), 798–806 (1998). [CrossRef]
  16. A. A. Fotiadi, O. L. Antipov, and P. Mégret, “Dynamics of pump-induced refractive index changes in single-mode Yb-doped optical fibers,” Opt. Express16(17), 12658–12663 (2008). [CrossRef] [PubMed]
  17. C. J. Corcoran and F. Durville, “Passive phasing in a coherent laser array,” IEEE J. Sel. Top. Quantum Electron.15(2), 294–300 (2009). [CrossRef]

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